A practical limitation in radio frequency (RF) wireless communication systems results from a mismatch between an in-phase (I) and quadrature (Q) phase component of a carrier signal, which is known as IQ imbalance. The carrier signal typically carries two independent bit streams, one on each of the in-phase (I) and quadrature (Q) components and these become imbalanced due to hardware imperfections encountered during signal processing. For example, a filter may introduce a phase shift or change in amplitude for one component but not the other. The resulting imbalance must be compensated for in order to achieve good performance. This is especially true for wideband systems, where the IQ imbalance becomes frequency dependent and needs to be carefully estimated in order to be compensated for.
In a communication system, transmitters (TX) and receivers (RX) will each give rise to an IQ imbalance and, in order to compensate for these, both the TX IQ imbalance and the RX IQ imbalance need to be estimated individually. However it is difficult to separate them as a received signal will have been affected by both the TX IQ imbalance and the RX IQ imbalance. This is especially difficult in light of carrier-frequency offset (CFO) which is always present in such systems due to a frequency mismatch between the transmitter and receiver oscillators.
It is known to transmit a particular periodic training sequence such that the RX IQ imbalance and CFO can be estimated and compensated for. After that, the TX IQ imbalance can be estimated from the compensated signal. However, a problem with existing schemes is that if the CFO is small (or zero), there will be a significant noise enhancement and known algorithms will not be able to accurately separate the IQ imbalance coming from the transmitter from that of the receiver.
Arrangements of certain embodiments will be understood and appreciated more fully from the following detailed description, made by way of example only and taken in conjunction with the drawings in which: